Such recorders include one or more magnetic read/write heads fixed to the periphery of a rotary component disposed to rotate about the axis of a cylindrical drum which has a magnetic tape wound helically around the surface thereof, with said tape running from a pay-out spool to a take-up spool. Information is recorded on the tape in sloping tracks with the angle of slope being a function of the ratio between the respective speeds of the tape and of the head. There are two families of recorders of this type. In the first family the heads are fixed to the periphery of a rotating platform which is coaxial with the drum and which is disposed in a slot provided in an equatorial plane of the drum, and said magnetic tape wound helically around the drum runs past said slot. In the other family, the drum comprises a stationary bottom portion and a rotating top portion with the heads fixed to the base thereof.
In order to convey information between the rotating heads and the associated (stationary) means of the recorder for processing or transmitting said information (for reading or recording purposes), each head is provided with a rotary electric coupling device between the stationary portion and the rotating portions (head-carrier platform) of the recorder.
Conventionally, the coupling device is constituted for each head by a rotating transformer comprising a rotor (associated with the head-carrier platform) and a (stationary) stator, both of which are cylindrical or annular in shape. The facing faces of the rotor and the stator are each provided with a groove containing a coil of conducting wire, thereby respectively constituting a (stator) primary winding and a (rotor) secondary winding.
Such prior devices suffer from drawbacks.
Because of the small sizes of the grooves in the stator and the rotor, and because of the small diameter of the electrically conducting wire used, putting the windings into place in the groove is complex and difficult. The resulting structure must have small mechanical clearances and any departure from the design dimensions has an effect on signal transmission.
Further, recorders of this type must be capable of reading and writing data at data throughput rates which can vary over a wide range (for example in a ratio of 1 to 25), whereas the format and the slope of the tracks on the tape is fixed (and set by standards, for example). Such different data rates imply that different head-to-tape relative speeds should be used, and as a result these speeds must also vary over a wide range.
This requires as a wide signal frequency band.
Unfortunately, prior rotating transformers put a limit on this frequency band.
Resonance phenomena in the equivalent circuit constituted by the head impedance and the leakage impedance in parallel on various stray capacitances have the effect of establishing an upper limit on the frequency band. Further, the circuit constituted by the output resistance of the amplifier connected to the transformer and the self-inductance of the transformer gives rise to a low cut-off frequency (depending on the value of said inductance) which applies a bottom limit to the frequency band.
The larger the ratio of the self-impedance over the leakage impedance (of the transformer), the wider the frequency passband.
For example, the frequency range obtained by prior transformers is at best about 100 kHz to 30 MHz (or 40 MHz). This range is not wide enough for satisfying all needs, given the available speed range, for which a frequency range of 3 kHz to 200 MHz (to -3 dB) would be necessary.
In addition, the mutual impedance of the transformer is difficult to control over the desired frequency range, in particular because it is difficult to control the characteristics of the ferrite used for making the transformer.
Finally, in order to verify write operations, a read operation accompanied by error correction is performed immediately after the write operation. Unfortunately the respective signal levels associated with these two operations are different: recording requires a high level signal whereas reading only gives rise to low level signals. Thus, the dynamic range over which such tranformers are capable of transmitting must be high, thereby increasing difficulties.
It is conventional practive to make use of auxiliary circuits to attempt to solve, or at least mitigate, these difficulties related to the use of rotating transformers.
In a first prior solution, the head-to-tape relative speed is fixed and the recorder includes a buffer memory containing an amount of data which varies as a function of time and which is related to variations in data throughput.
Although this technique makes it possible to reduce the frequency range to a range which is compatible with the frequency band limits applicable to prior rotating transformers (as indicated above), it does nothing to solve the difficulties related to the differences in level between recording and reading (also mentioned above). Further, this technique complicates the structure of the recorder by requiring the additional presence of the buffer memory whose capacity must be increased for increasing variations over time in data throughput.
A second solution consists in placing an electronic amplifier circuit on the head-carrier platform for each head. Each circuit is disposed between the head and the rotating transformer associated therewith. This provides read and write signals at compatible levels since the amplifiers can be used to reduce the level required for the recording signals and to increase the level of the read signals. Crosstalk is reduced by bringing these signal levels closer together. In addition, the passband is widened since the magnetic head is now isolated from the imperfect coupler constituted by the rotating transformer and it is therefore possible to give the self-inductance of the transformer a value such that the ratio of its self-impedance over the transformer leakage impedance is increased. In any event, because of the amplifier circuit on the platform, this ratio has less influence than in other prior solutions which do not include an amplifier circuit on the platform.
However, the opportunities for increasing this ratio are limited. This prior circuit thus gives rise to a frequency band which, although wider, nevertheless suffers from upper and lower limits which are too close together.
Thus, it will be understood that these two prior solutions solve the above-mentioned problems in part, only. Further, neither of them escapes from the defect of the transformer being complex to make, since both of them require a transformer.
The present invention remedies these drawbacks and proposes a coupling device which is simple to make, which makes a wide frequency band possible, and which is unaffected by differences in signal level between recording and reading.